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author Franz Glasner <fzglas.hg@dom66.de>
date Mon, 15 Sep 2025 11:43:07 +0200
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1 The Independent JPEG Group's JPEG software
2 ==========================================
3
4 README for release 9f of 14-Jan-2024
5 ====================================
6
7 This distribution contains the ninth public release of the Independent JPEG
8 Group's free JPEG software. You are welcome to redistribute this software and
9 to use it for any purpose, subject to the conditions under LEGAL ISSUES, below.
10
11 This software is the work of Tom Lane, Guido Vollbeding, Philip Gladstone,
12 Bill Allombert, Jim Boucher, Lee Crocker, Bob Friesenhahn, Ben Jackson,
13 John Korejwa, Julian Minguillon, Luis Ortiz, George Phillips, Davide Rossi,
14 Ge' Weijers, and other members of the Independent JPEG Group.
15
16 IJG is not affiliated with the ISO/IEC JTC1/SC29/WG1 standards committee
17 (previously known as JPEG, together with ITU-T SG16).
18
19
20 DOCUMENTATION ROADMAP
21 =====================
22
23 This file contains the following sections:
24
25 OVERVIEW General description of JPEG and the IJG software.
26 LEGAL ISSUES Copyright, lack of warranty, terms of distribution.
27 REFERENCES Where to learn more about JPEG.
28 ARCHIVE LOCATIONS Where to find newer versions of this software.
29 ACKNOWLEDGMENTS Special thanks.
30 FILE FORMAT WARS Software *not* to get.
31 TO DO Plans for future IJG releases.
32
33 Other documentation files in the distribution are:
34
35 User documentation:
36 install.txt How to configure and install the IJG software.
37 usage.txt Usage instructions for cjpeg, djpeg, jpegtran,
38 rdjpgcom, and wrjpgcom.
39 *.1 Unix-style man pages for programs (same info as usage.txt).
40 wizard.txt Advanced usage instructions for JPEG wizards only.
41 cdaltui.txt Description of alternate user interface for cjpeg/djpeg.
42 change.log Version-to-version change highlights.
43 Programmer and internal documentation:
44 libjpeg.txt How to use the JPEG library in your own programs.
45 example.c Sample code for calling the JPEG library.
46 structure.txt Overview of the JPEG library's internal structure.
47 filelist.txt Road map of IJG files.
48 coderules.txt Coding style rules --- please read if you contribute code.
49
50 Please read at least the files install.txt and usage.txt. Some information
51 can also be found in the JPEG FAQ (Frequently Asked Questions) article. See
52 ARCHIVE LOCATIONS below to find out where to obtain the FAQ article.
53
54 If you want to understand how the JPEG code works, we suggest reading one or
55 more of the REFERENCES, then looking at the documentation files (in roughly
56 the order listed) before diving into the code.
57
58
59 OVERVIEW
60 ========
61
62 This package contains C software to implement JPEG image encoding, decoding,
63 and transcoding. JPEG (pronounced "jay-peg") is a standardized compression
64 method for full-color and grayscale images.
65
66 This software implements JPEG baseline, extended-sequential, and progressive
67 compression processes. Provision is made for supporting all variants of these
68 processes, although some uncommon parameter settings aren't implemented yet.
69 We have made no provision for supporting the hierarchical or lossless
70 processes defined in the standard.
71
72 We provide a set of library routines for reading and writing JPEG image files,
73 plus two sample applications "cjpeg" and "djpeg", which use the library to
74 perform conversion between JPEG and some other popular image file formats.
75 The library is intended to be reused in other applications.
76
77 In order to support file conversion and viewing software, we have included
78 considerable functionality beyond the bare JPEG coding/decoding capability;
79 for example, the color quantization modules are not strictly part of JPEG
80 decoding, but they are essential for output to colormapped file formats or
81 colormapped displays. These extra functions can be compiled out of the
82 library if not required for a particular application.
83
84 We have also included "jpegtran", a utility for lossless transcoding between
85 different JPEG processes, and "rdjpgcom" and "wrjpgcom", two simple
86 applications for inserting and extracting textual comments in JFIF files.
87
88 The emphasis in designing this software has been on achieving portability and
89 flexibility, while also making it fast enough to be useful. In particular,
90 the software is not intended to be read as a tutorial on JPEG. (See the
91 REFERENCES section for introductory material.) Rather, it is intended to
92 be reliable, portable, industrial-strength code. We do not claim to have
93 achieved that goal in every aspect of the software, but we strive for it.
94
95 We welcome the use of this software as a component of commercial products.
96 No royalty is required, but we do ask for an acknowledgement in product
97 documentation, as described under LEGAL ISSUES.
98
99
100 LEGAL ISSUES
101 ============
102
103 In plain English:
104
105 1. We don't promise that this software works. (But if you find any bugs,
106 please let us know!)
107 2. You can use this software for whatever you want. You don't have to pay us.
108 3. You may not pretend that you wrote this software. If you use it in a
109 program, you must acknowledge somewhere in your documentation that
110 you've used the IJG code.
111
112 In legalese:
113
114 The authors make NO WARRANTY or representation, either express or implied,
115 with respect to this software, its quality, accuracy, merchantability, or
116 fitness for a particular purpose. This software is provided "AS IS", and you,
117 its user, assume the entire risk as to its quality and accuracy.
118
119 This software is copyright (C) 1991-2024, Thomas G. Lane, Guido Vollbeding.
120 All Rights Reserved except as specified below.
121
122 Permission is hereby granted to use, copy, modify, and distribute this
123 software (or portions thereof) for any purpose, without fee, subject to these
124 conditions:
125 (1) If any part of the source code for this software is distributed, then this
126 README file must be included, with this copyright and no-warranty notice
127 unaltered; and any additions, deletions, or changes to the original files
128 must be clearly indicated in accompanying documentation.
129 (2) If only executable code is distributed, then the accompanying
130 documentation must state that "this software is based in part on the work of
131 the Independent JPEG Group".
132 (3) Permission for use of this software is granted only if the user accepts
133 full responsibility for any undesirable consequences; the authors accept
134 NO LIABILITY for damages of any kind.
135
136 These conditions apply to any software derived from or based on the IJG code,
137 not just to the unmodified library. If you use our work, you ought to
138 acknowledge us.
139
140 Permission is NOT granted for the use of any IJG author's name or company name
141 in advertising or publicity relating to this software or products derived from
142 it. This software may be referred to only as "the Independent JPEG Group's
143 software".
144
145 We specifically permit and encourage the use of this software as the basis of
146 commercial products, provided that all warranty or liability claims are
147 assumed by the product vendor.
148
149
150 The Unix configuration script "configure" was produced with GNU Autoconf.
151 It is copyright by the Free Software Foundation but is freely distributable.
152 The same holds for its supporting scripts (config.guess, config.sub,
153 ltmain.sh). Another support script, install-sh, is copyright by X Consortium
154 but is also freely distributable.
155
156
157 REFERENCES
158 ==========
159
160 We recommend reading one or more of these references before trying to
161 understand the innards of the JPEG software.
162
163 The best short technical introduction to the JPEG compression algorithm is
164 Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
165 Communications of the ACM, April 1991 (vol. 34 no. 4), pp. 30-44.
166 (Adjacent articles in that issue discuss MPEG motion picture compression,
167 applications of JPEG, and related topics.) If you don't have the CACM issue
168 handy, a PDF file containing a revised version of Wallace's article is
169 available at https://www.ijg.org/files/Wallace.JPEG.pdf. The file (actually
170 a preprint for an article that appeared in IEEE Trans. Consumer Electronics)
171 omits the sample images that appeared in CACM, but it includes corrections
172 and some added material. Note: the Wallace article is copyright ACM and IEEE,
173 and it may not be used for commercial purposes.
174
175 A somewhat less technical, more leisurely introduction to JPEG can be found in
176 "The Data Compression Book" by Mark Nelson and Jean-loup Gailly, published by
177 M&T Books (New York), 2nd ed. 1996, ISBN 1-55851-434-1. This book provides
178 good explanations and example C code for a multitude of compression methods
179 including JPEG. It is an excellent source if you are comfortable reading C
180 code but don't know much about data compression in general. The book's JPEG
181 sample code is far from industrial-strength, but when you are ready to look
182 at a full implementation, you've got one here...
183
184 The best currently available description of JPEG is the textbook "JPEG Still
185 Image Data Compression Standard" by William B. Pennebaker and Joan L.
186 Mitchell, published by Van Nostrand Reinhold, 1993, ISBN 0-442-01272-1.
187 Price US$59.95, 638 pp. The book includes the complete text of the ISO JPEG
188 standards (DIS 10918-1 and draft DIS 10918-2).
189 Although this is by far the most detailed and comprehensive exposition of
190 JPEG publicly available, we point out that it is still missing an explanation
191 of the most essential properties and algorithms of the underlying DCT
192 technology.
193 If you think that you know about DCT-based JPEG after reading this book,
194 then you are in delusion. The real fundamentals and corresponding potential
195 of DCT-based JPEG are not publicly known so far, and that is the reason for
196 all the mistaken developments taking place in the image coding domain.
197
198 The original JPEG standard is divided into two parts, Part 1 being the actual
199 specification, while Part 2 covers compliance testing methods. Part 1 is
200 titled "Digital Compression and Coding of Continuous-tone Still Images,
201 Part 1: Requirements and guidelines" and has document numbers ISO/IEC IS
202 10918-1, ITU-T T.81. Part 2 is titled "Digital Compression and Coding of
203 Continuous-tone Still Images, Part 2: Compliance testing" and has document
204 numbers ISO/IEC IS 10918-2, ITU-T T.83.
205 IJG JPEG 8 introduced an implementation of the JPEG SmartScale extension
206 which is specified in two documents: A contributed document at ITU and ISO
207 with title "ITU-T JPEG-Plus Proposal for Extending ITU-T T.81 for Advanced
208 Image Coding", April 2006, Geneva, Switzerland. The latest version of this
209 document is Revision 3. And a contributed document ISO/IEC JTC1/SC29/WG1 N
210 5799 with title "Evolution of JPEG", June/July 2011, Berlin, Germany.
211 IJG JPEG 9 introduces a reversible color transform for improved lossless
212 compression which is described in a contributed document ISO/IEC JTC1/SC29/
213 WG1 N 6080 with title "JPEG 9 Lossless Coding", June/July 2012, Paris, France.
214
215 The JPEG standard does not specify all details of an interchangeable file
216 format. For the omitted details we follow the "JFIF" conventions, version 2.
217 JFIF version 1 has been adopted as Recommendation ITU-T T.871 (05/2011) :
218 Information technology - Digital compression and coding of continuous-tone
219 still images: JPEG File Interchange Format (JFIF). It is available as a
220 free download in PDF file format from https://www.itu.int/rec/T-REC-T.871.
221 A PDF file of the older JFIF document is available at
222 https://www.w3.org/Graphics/JPEG/jfif3.pdf.
223
224 The TIFF 6.0 file format specification can be obtained by FTP from
225 ftp://ftp.sgi.com/graphics/tiff/TIFF6.ps.gz. The JPEG incorporation scheme
226 found in the TIFF 6.0 spec of 3-June-92 has a number of serious problems.
227 IJG does not recommend use of the TIFF 6.0 design (TIFF Compression tag 6).
228 Instead, we recommend the JPEG design proposed by TIFF Technical Note #2
229 (Compression tag 7). Copies of this Note can be obtained from
230 https://www.ijg.org/files/. It is expected that the next revision
231 of the TIFF spec will replace the 6.0 JPEG design with the Note's design.
232 Although IJG's own code does not support TIFF/JPEG, the free libtiff library
233 uses our library to implement TIFF/JPEG per the Note.
234
235
236 ARCHIVE LOCATIONS
237 =================
238
239 The "official" archive site for this software is www.ijg.org.
240 The most recent released version can always be found there in
241 directory "files". This particular version will be archived
242 in Windows-compatible "zip" archive format as
243 https://www.ijg.org/files/jpegsr9f.zip, and
244 in Unix-compatible "tar.gz" archive format as
245 https://www.ijg.org/files/jpegsrc.v9f.tar.gz.
246
247 The JPEG FAQ (Frequently Asked Questions) article is a source of some
248 general information about JPEG.
249 It is available on the World Wide Web at http://www.faqs.org/faqs/jpeg-faq/
250 and other news.answers archive sites, including the official news.answers
251 archive at rtfm.mit.edu: ftp://rtfm.mit.edu/pub/usenet/news.answers/jpeg-faq/.
252 If you don't have Web or FTP access, send e-mail to mail-server@rtfm.mit.edu
253 with body
254 send usenet/news.answers/jpeg-faq/part1
255 send usenet/news.answers/jpeg-faq/part2
256
257
258 ACKNOWLEDGMENTS
259 ===============
260
261 Thank to Juergen Bruder for providing me with a copy of the common DCT
262 algorithm article, only to find out that I had come to the same result
263 in a more direct and comprehensible way with a more generative approach.
264
265 Thank to Istvan Sebestyen and Joan L. Mitchell for inviting me to the
266 ITU JPEG (Study Group 16) meeting in Geneva, Switzerland.
267
268 Thank to Thomas Wiegand and Gary Sullivan for inviting me to the
269 Joint Video Team (MPEG & ITU) meeting in Geneva, Switzerland.
270
271 Thank to Thomas Richter and Daniel Lee for inviting me to the
272 ISO/IEC JTC1/SC29/WG1 (previously known as JPEG, together with ITU-T SG16)
273 meeting in Berlin, Germany.
274
275 Thank to John Korejwa and Massimo Ballerini for inviting me to
276 fruitful consultations in Boston, MA and Milan, Italy.
277
278 Thank to Hendrik Elstner, Roland Fassauer, Simone Zuck, Guenther
279 Maier-Gerber, Walter Stoeber, Fred Schmitz, and Norbert Braunagel
280 for corresponding business development.
281
282 Thank to Nico Zschach and Dirk Stelling of the technical support team
283 at the Digital Images company in Halle for providing me with extra
284 equipment for configuration tests.
285
286 Thank to Richard F. Lyon (then of Foveon Inc.) for fruitful
287 communication about JPEG configuration in Sigma Photo Pro software.
288
289 Thank to Andrew Finkenstadt for hosting the ijg.org site.
290
291 Thank to Thomas G. Lane for the original design and development
292 of this singular software package.
293
294 Thank to Lars Goehler, Andreas Heinecke, Sebastian Fuss,
295 Yvonne Roebert, Andrej Werner, Ulf-Dietrich Braumann,
296 and Nina Ssymank for support and public relations.
297
298
299 FILE FORMAT WARS
300 ================
301
302 The ISO/IEC JTC1/SC29/WG1 standards committee (previously known as JPEG,
303 together with ITU-T SG16) currently promotes different formats containing
304 the name "JPEG" which is misleading because these formats are incompatible
305 with original DCT-based JPEG and are based on faulty technologies.
306 IJG therefore does not and will not support such momentary mistakes
307 (see REFERENCES).
308 There exist also distributions under the name "OpenJPEG" promoting such
309 kind of formats which is misleading because they don't support original
310 JPEG images.
311 We have no sympathy for the promotion of inferior formats. Indeed, one of
312 the original reasons for developing this free software was to help force
313 convergence on common, interoperable format standards for JPEG files.
314 Don't use an incompatible file format!
315 (In any case, our decoder will remain capable of reading existing JPEG
316 image files indefinitely.)
317
318 The ISO committee pretends to be "responsible for the popular JPEG" in their
319 public reports which is not true because they don't respond to actual
320 requirements for the maintenance of the original JPEG specification.
321 Furthermore, the ISO committee pretends to "ensure interoperability" with
322 their standards which is not true because their "standards" support only
323 application-specific and proprietary use cases and contain mathematically
324 incorrect code.
325
326 There are currently different distributions in circulation containing the
327 name "libjpeg" which is misleading because they don't have the features and
328 are incompatible with formats supported by actual IJG libjpeg distributions.
329 One of those fakes is released by members of the ISO committee and just uses
330 the name of libjpeg for misdirection of people, similar to the abuse of the
331 name JPEG as described above, while having nothing in common with actual IJG
332 libjpeg distributions and containing mathematically incorrect code.
333 The other one claims to be a "derivative" or "fork" of the original libjpeg,
334 but violates the license conditions as described under LEGAL ISSUES above
335 and violates basic C programming properties.
336 We have no sympathy for the release of misleading, incorrect and illegal
337 distributions derived from obsolete code bases.
338 Don't use an obsolete code base!
339
340 According to the UCC (Uniform Commercial Code) law, IJG has the lawful and
341 legal right to foreclose on certain standardization bodies and other
342 institutions or corporations that knowingly perform substantial and
343 systematic deceptive acts and practices, fraud, theft, and damaging of the
344 value of the people of this planet without their knowing, willing and
345 intentional consent.
346 The titles, ownership, and rights of these institutions and all their assets
347 are now duly secured and held in trust for the free people of this planet.
348 People of the planet, on every country, may have a financial interest in
349 the assets of these former principals, agents, and beneficiaries of the
350 foreclosed institutions and corporations.
351 IJG asserts what is: that each man, woman, and child has unalienable value
352 and rights granted and deposited in them by the Creator and not any one of
353 the people is subordinate to any artificial principality, corporate fiction
354 or the special interest of another without their appropriate knowing,
355 willing and intentional consent made by contract or accommodation agreement.
356 IJG expresses that which already was.
357 The people have already determined and demanded that public administration
358 entities, national governments, and their supporting judicial systems must
359 be fully transparent, accountable, and liable.
360 IJG has secured the value for all concerned free people of the planet.
361
362 A partial list of foreclosed institutions and corporations ("Hall of Shame")
363 is currently prepared and will be published later.
364
365
366 TO DO
367 =====
368
369 Version 9 is the second release of a new generation JPEG standard
370 to overcome the limitations of the original JPEG specification,
371 and is the first true source reference JPEG codec.
372 More features are being prepared for coming releases...
373
374 Please send bug reports, offers of help, etc. to jpeg-info@ijg.org.